How can you determine if a compound is a purine or pyrimidine?

A pyrimidine has one ring; a purine has two rings.

The pyrimidines (##bb"C"##ytosine, ##bb"U"##racil, and ##bb"T"##hymine) all have one ring.

The purines have two rings.

One way to remember the difference is:

The pyrimidines are C U T from the purines.

If they were no labels in the picture below, could you identify the purines and the pyrimidines?